Neurologic, and neurologically related otologic, and ophthalmologic diseases can be very debilitating and/or fatal to the patient, whether induced by various forms of disease or stroke, or various forms of trauma, because the repair and/or regeneration of damaged neurons is often difficult or impossible. Various naturally occurring protease enzymes are believed to be part of the basic machinery of cellular catabolism and part of the mechanism of neuronal cell death pathways (see Nakanishi et al., reference 28).
Although much research has been focused on the prevention and treatment of such neurologic, and neurologically related otologic, and ophthalmologic diseases or injuries, whether caused by trauma, stroke, ischemia, or various immunological and/or degenerative disease processes, the successful development of treatments for such neurologically related diseases have been limited, and there remains a long-felt and unmet need in the art for improved treatments of such neurologically related diseases.
Calcium-activated neutral proteases (“Calpains”) refers to a family of structurally related heterodimeric neutral and non-lysosomal Ca2+ activated cysteine proteases that are found in all tissue and cell types. Calpains are believed to be involved in important signaling pathways, modulation of enzyme activity, processing of hormones, protein turnover, and cytoskeletal rearrangements. See “The Calpain System” by Goll et al., (59), and Ray et al., (67). Calpains are typically cytosolic proteolytic enzymes that become activated against a number of cytosolic and membrane proteins, cytokines, transcriptional factors, kinases, phosphatases, and lens proteins, when increased levels of intracellular free Ca2+ions are present (micromolar levels for “μ-calpains” and millimolar concentrations for “m-calpains”). Calpains are also believed to be involved in many cellular death processes initiated by various traumas, ischemia, and immunological and inflammation-induced diseases, as well as a variety of central nervous system diseases wherein various cellular and/or membrane defects and/or injuries affect the cellular homeostasis of calcium ions, leading to increased concentrations of intracellular calcium ions and the activation of the calpains. Calpains are believed to be involved in a variety of degenerative diseases of the central nervous system, including stroke and related ischemic diseases, spinal cord injuries, traumatic brain injuries, retinal degeneration, cataracts, antibiotic-induced ototoxicity, Alzheimer's disease, amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease), Huntington's disease, Parkinson's disease, spinocerebellar atrophies, multiple sclerosis, inflammatory demyelinating polyneuropathy, and other polyneuropathies.
One degenerative neurologic disease that is particularly devastating is multiple sclerosis (“MS”). Multiple sclerosis is an autoimmune, neurodegenerative disease that affects about 350,000 people in the United States and is a major cause of nervous system disability or death in young adults (see references 11, 14, and 23). A common clinical condition in humans afflicted with MS is inflammation of the central nervous system (“CNS”) white matter, and the degenerative formation of neural lesions resulting from extensive degradation of the myelin sheaths surrounding the axons of the neurons, and eventual degradation of the axons themselves. The demyelination that occurs in MS is believed to be initiated by the attack of protease enzymes on three major neurological proteins: myelin basic protein (MBP), proteo-lipid protein (PLP) and myelin oligodendrocyte glycoprotein (MOG). Mechanistically, MS is an inflammatory demyelinating disease that is at least partially caused by an autoimmune response to myelin degradation products (6, 7, 13, 24, 26, 32, 48, 55, 56). Recent studies have emphasized the role of axonal injury in addition to the well known demyelation and inflammatory mechanisms.
Peptides derived from the MBP, PLP, and MOG neurological proteins can have the properties of encephalitogenic epitopes, and can be used to induce experimental autoimmune encephalomyelitis (EAE) in mice, which is well known as a valuable animal model for MS (1, 9, 12, 18, 24, 25, 29, 43, 46, 47, 52, 45, 55). A combination of adoptive transfer of MBP-specific T-cells (26,32, 57) and/or active immunization of mice with encephalitogenic peptide derived from MOG, often in the presence of immunogenic adjuvants (17, 24, 27) initiate an immune response and result in the demyelination of neurons in the mice, to provide the EAE animal model of the similar demyelination observed during MS. Activated T-cells are believed to constitute only a small fraction of the total inflammatory cells in the central nervous system of mice with acute EAE, or humans with MS. The T-cells recruit a large population of macrophage cells. In addition, the microglia, which serve as macrophages in the brain and spinal cord of the central nervous system, are also activated. The macrophages and microglia present in both EAE and MS induce damage to the myelin of neurons in large part via their proteolytic activity.
Calpain is activated in the CNS tissue of animals with EAE (8, 44, 45), in patients with MS (33, 34), and during myelinolysis (28, 35-37, 40, 41, 43). Calpain is significantly elevated in the white matter of MS patients and is increasingly expressed in activated microglia and macrophages in MS patients. Although other proteases may also be involved in myelin degradation, recent studies suggest that calpains play a major role in this process in the EAE model (22, 30, 26-39, 41, 43, 46, 47). Inhibition of calpain activity should limit the ultimate pathology in both EAE and MS and other diseases involving proteolytic demyelination and neurodegeration.
Some studies have demonstrated calpain inhibition with the tripeptide protease inhibitor leupeptin in peripheral nerve (2-6), and leupeptin's effect in suppressing EAE in rats (29). It has also been shown that leupeptin protects cochlear and vestibular hair from gentamycin oxotoxicity (6, 10). Leupeptin's structure is shown below:

However, among the limitations of leupeptin in these clinical applications is its poor cellular permeability, including a poor ability to cross the blood brain barrier (BBB) to reach desired sites of action within the central nervous system (CNS). Leupeptin can be transported across the BBB and into the CNS by liposome encapsulation (29), but this method is technically difficult and impractical for routine use. Moreover, leupeptin and other known small molecule protease inhibitors are not very selective, since they typically inhibit the proteases in many non-diseased tissues in the body, including serine proteases. Therefore, a method of treatment of MS and other nervous and central nervous system degenerative diseases wherein therapeutic agents (such as calpain inhibitors) are efficiently transferred across the BBB and targeted to their site of action within the central nervous system is highly desirable.
Taurine (2-aminethanesulfonic acid, structure shown below) is the most abundant free amino acid in many tissues including leukocytes and brain, where it is present in millimolar concentrations (15, 16, 31, 34, 42). The brain synthesizes only limited amounts of taurine and thus significant amounts must be transported into those parts of the brain and central nervous system that require it (19). Taurine is believed to be transported into cells via a Na+-dependent transport system (16), and two distinct Na+-dependent high affinity taurine transporters have been cloned.

Cysteic acid (α-amino-β-sulfo-propionic acid) shares structural similarities with taurine. It is a competitive inhibitor of taurine transport and thus utilizes the same transport mechanisms (20, 21). Cysteic acid has a carboxyl group in addition to the sulfonic acid and amino groups of taurine, which provides another functional group to which other protease inhibitor residues can be attached. U.S. Pat. Nos. 4,866,040, and 5,008,288 disclosed the synthesis of a compound in which a leucyl-argininal residue was attached to cysteic acid, and disclosed that such compounds could “be useful for specific delivery to a variety of tissues such as cardiac and skeletal muscle, nervous tissue, adrenal medulla, platelets, etc.” Those patents did not, however, further elaborate on the specific neurological disorders to be treated, provide any details how such treatments should be carried out, or demonstrate that any such speculated treatments actually work successfully. These patents also did not elaborate on the potential advantages of using a calpain inhibitor conjugated to cysteic acid for the purpose of treating neurological diseases triggered by an autoimmune response, such as multiple sclerosis. In such autoimmune neurodegenerative diseases, the enhanced uptake of cysteic acid conjugates by leukocytes may provide an unexpected benefit, by inhibiting active calpain that is released into diseased tissue as part of the undesired immune response, in addition to inhibiting calpain activity within neurons.
The development of improved small molecule calpain inhibitors has been the subject of a good deal of research (see Ray et al., 67), but “there has been no report yet showing the effectiveness of these inhibitors for inhibition of calpain in vivo,” and “It should be emphasized that there are difficulties in developing small molecule inhibitors with the appropriate drug properties. Although calpain is an important drug discovery target, it has been proven to be a difficult one that may or may not be tractable.” Thus, many have tried and failed to successfully develop calpain inhibitors, and there remains in the art a long felt and as yet unsatisfied need for the development of protease inhibitors that can be targeted to specific tissues to treat or prevent specific diseases, such as. stroke and related ischemic diseases, spinal cord injuries, traumatic brain injuries, retinal degeneration, cataracts, acoustic trauma due to loud noises, antibiotic-induced ototoxicity, multiple sclerosis, Alzheimer's disease, amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease), Huntington's disease, Parkinson's disease, spinocerebellar atrophies and inflammatory demyelinating polyneuropathy.
To overcome these problems in the art, described herein below are compounds that facilitate the selective transport and/or concentration of protease inhibitors that can be bound thereto, such as, for example, calpain inhibitors, within nervous tissue involved in specific neurologically-related diseases.